This invention relates to a process for improving the purity of an aqueous onium hydroxide solution. In particular, the invention relates to a process for improving the purity of aqueous onium hydroxide solutions containing undesirable amounts of anions. The invention also relates to the improved high purity onium hydroxide solutions obtained by the above method.
Onium hydroxides, such as quaternary ammonium hydroxides including tetramethylammonium hydroxide (TMAH) and tetraethylammonium hydroxide (TEAH), are strong organic bases that have been known for many years. Quaternary ammonium hydroxides have found a variety of uses including use in zeolite manufacture and polymer manufacture. Aqueous solutions of quaternary ammonium hydroxides, particularly TMAH solutions, have also been used extensively as a developer for photoresists in printed circuit board and microelectronic chip fabrication. For a variety of reasons, it is desirable to minimize the overall amount of developer used in printed circuit board and microelectronic chip fabrication. One way to minimize the overall amount of hydroxide developer is to reuse the waste developer. Reusing developer reduces the amount lost and decreases disposal problems.
Waste developer contains impurities including ionic impurities and nonionic impurities. Ionic impurities include various metal cations such as sodium, potassium, zinc, nickel, aluminum, copper and calcium; and anions such as halides, nitrates, nitrites, carbonates, carboxylates, sulfates. Nonionic impurities include photoresists, surfactants, amines and numerous other organic molecules. Waste developer also contains relatively low concentrations of the hydroxide developer. Accordingly, there remains a continuing need to effectively recover hydroxide developer in a useable form so that it may be reused thereby minimize the overall amount of developer used in printed circuit board and microelectronic chip fabrication.
U.S. Pat. No. 4,714,530 (Hale et al) describes an electrolytic process for preparing high purity quaternary ammonium hydroxides which utilizes a cell containing a catholyte compartment and an anolyte compartment separated by a cation-exchange membrane. The process comprises charging an aqueous solution of a quaternary ammonium hydroxide to the anolyte compartment, adding water to the catholyte compartment, and passing a direct current through the electrolysis cell to produce a higher purity quaternary ammonium hydroxide in the catholyte compartment which is subsequently recovered. The ""530 patent also describes an improvement which comprises heating the quaternary ammonium hydroxide at an elevated temperature prior to charging the hydroxide to the anolyte compartment of the electrolytic cell.
U.S. Pat. No. 4,938,854 (Sharifian et al) also describes an electrolytic process for purifying quaternary ammonium hydroxides by lowering the latent halide content. The electrolytic cell may be divided into an anolyte compartment and a catholyte compartment by a divider which may be an anion or cation selective membrane. The cathode in the catholyte compartment comprises zinc, cadmium, tin, lead, copper or titanium, or alloys thereof, mercury or mercury amalgam.
Japanese Kokai Patent No. 60-131985 (1985) (Takahashi et al) describes a method of manufacturing a high purity quaternary ammonium hydroxide in an electrolysis cell which is divided into an anode chamber and a cathode chamber by a cation exchange membrane. A quaternary ammonium hydroxide solution containing impurities is charged to the anode chamber and a direct current is supplied between two electrodes after water has been charged to the cathode chamber. Purified quaternary ammonium hydroxide is obtained from the cathode chamber. The purified quaternary ammonium hydroxide contains reduced amounts of alkali metals, alkaline earth metals, anions, etc.
U.S. Pat. Nos. 5,439,564 and 5,545,309 (Shimizu et al) relate to methods of processing waste liquid containing an organic quaternary ammonium hydroxide by contacting the waste liquid with a cation-exchanging material, eluting organic quaternary ammonium cations from the cation-exchanging material, and electrolyzing the eluate in a two chamber electrolytic cell equipped with an anode, cathode and cation-exchanging membrane. Organic quaternary ammonium hydroxide is obtained from the cathode chamber of the electrolytic cell.
U.S. Pat. No. 5,968,338 (Hulme et al) describes a process for regenerating onium hydroxides from solutions containing onium compounds such as hydroxides and salts using a cation exchange material, an acid to form a salt, and an electrochemical cell comprising at least three compartments to regenerate the onium hydroxide.
U.S. Pat. No. 6,207,039 (Moulton et al) describes a process for recovering an onium hydroxide or onium salt from a solution containing an onium compound (e.g., a hydroxide, salt, or mixture thereof) using an electrochemical cell comprising at least two compartments, a cathode, an anode, and a divider, wherein at least one compartment contains an ion exchange material. A variety of electrochemical cells are described. In one cell illustrated in FIG. 3, the cell comprises an anode, a cathode and a unit cell containing in sequence beginning at the anode, a first bipolar membrane, a cation selective membrane and a second bipolar membrane. The compartment formed by the first bipolar membrane and the cation exchange membrane (feed compartment) contains an ion exchange material.
Another electrochemical cell which is illustrated in FIGS. 8 and 8A of the ""039 patent comprises an anode, a cathode, and a unit cell containing in sequence beginning at the anode, a first bipolar membrane, a first cation selective membrane, a second cation selective membrane, and a second bipolar membrane. The feed compartment in both embodiments contains an ion exchange material. In the embodiment of FIG. 8B, the feed compartment is formed by the first bipolar membrane and the first cation selective membrane.
In one embodiment, the present invention is a process for improving the purity of aqueous onium hydroxide solutions which comprises:
(A) providing an electrochemical cell comprising at least four compartments, said compartments being formed by a cathode, an anode, and in order from the anode to the cathode, a first bipolar membrane, a first cation selective membrane, and a second bipolar membrane,
(B) charging the onium hydroxide solution to a feed compartment formed by the first bipolar membrane and the first cation selective membrane wherein the feed compartment is free of any ion exchange material,
(C) passing a current through the cell, and
(D) recovering a purified onium hydroxide solution from the recovery compartment formed by the first cation selective membrane and the second bipolar membrane.
In another embodiment, the present invention relates to a process for improving the purity of an aqueous onium hydroxide solution which comprises:
(A) providing an electrochemical cell comprising at least 5 compartments, said compartments being formed by an anode, a cathode, and in order from the anode to the cathode, a first bipolar membrane, a first cation selective membrane, a second cation selective membrane, and a second bipolar membrane,
(B) charging the onium hydroxide solution to a feed compartment formed by the first bipolar membrane and the first cation selective membrane wherein the feed compartment is free of any ion exchange material,
(C) passing a current through the cell, and
(D) recovering a purified onium hydroxide from the recovery compartment formed by the second cation selective membrane and the second bipolar membrane.
The onium hydroxide solutions that can be purified by the processes of the invention include quaternary ammonium hydroxides, quaternary phosphonium hydroxides, and tertiary sulfonium hydroxides.
As a result of the process of the present invention, recycled solutions of onium hydroxides can be obtained in which the concentration and purity is increased. Recycling spent solutions of onium hydroxides provides not only cost savings, but also environmental benefits by eliminating or reducing the need for synthesizing new hydroxide compound solutions, the associated expensive purification processes, and the toxicity of waste solution effluents. In addition, it is not necessary to store large amounts of chemicals. The relatively high concentration and purity of the onium hydroxide solutions obtainable via the present invention can effectively be used in numerous applications where onium hydroxide solutions are required.